Radiation maps of ocean sediment from the Castle Bravo crater Emlyn W. Hughesa,b,1, Monica Rouco Molinaa, Maveric K. I. L. Abellaa, Ivana Nikolic-Hughesa,c, and Malvin A. Rudermanb,1 aK=1 Project, Center for Nuclear Studies, Columbia University, New York, NY 10027; bDepartment of Physics, Columbia University, New York, NY 10027; and cDepartment of Chemistry, Columbia University, New York, NY 10027 Contributed by Malvin A. Ruderman, May 14, 2019 (sent for review March 1, 2019; reviewed by David Kawall and Yury G. Kolomensky) On March 1, 1954, the United States conducted its largest comes from a complicated mix of nuclear weapons tests detonated thermonuclear weapon test in Bikini Atoll in the Marshall Islands; in the area. The range of yields was from 1 to 15 megatons for the the detonation was code-named “Castle Bravo.” Radioactive de- 7 nuclear weapons detonations in the Bravo crater vicinity (Table posits in the ocean sediment at the bomb crater are widespread 1 and Fig. 1). The total yield from nuclear-weapons tests per- and high levels of contamination remain today. One hundred formed in the few-kilometer-diameter Bravo crater region corre- thirty cores were collected from the top 25 cm of surface sediment sponds to one-third of the total yield from the entire US nuclear 2 at ocean depths approaching 60 m over a ∼2-km area, allowing for weapons testing program in the Marshall Islands. a presentation of radiation maps of the Bravo crater site. Radio- Over the years, sediment samples have been collected in Bikini chemical analyses were performed on the following radionuclides: Atoll to determine the radioactive contamination of the lagoon plutonium-(239,240), plutonium-238, americium-241, bismuth-207, from the nuclear weapons testing program. In the 1960s and and cesium-137. Large values of plutonium-(239,240), americium- 1970s, a handful of cores was collected in the Bravo crater and 241, and bismuth-207 are found. Comparisons are made to core the radionuclide presence was measured in these samples (5–7). sample results from other areas in the northern Marshall Islands. A recent study of ocean cores collected at the Bikini and Enewetak test sites provides an up-to-date quantitative result of Bravo crater | Bikini Island | cesium-137 | ocean sediment | plutonium radiation levels in the crater sediment (8). However, the study was limited to a few cores. No large-scale systematic study has n March 1, 1954, the US military detonated its largest been published on the radioactive contamination to ocean sed- Othermonuclear weapon on an island located in the north- iment from nuclear weapons testing. western rim of Bikini Atoll in the Marshall Islands. The weapon, Here, we present measurements of the radioactivity levels of 5 code-named Castle Bravo, released an energy equivalent to 15 radionuclides from 129 cores collected within the Bravo crater. million tons of trinitrotoluene (TNT) (15 megatons), a value These results are used to create radiation maps of the Bravo substantially larger than the US military’s prior estimates, al- crater, allowing for a more detailed and systematic discussion of though controversy still remains on this topic (1). [For example, radioactive contamination from thermonuclear weapons testing. declassified documents show that the routing and placement of observing aircraft were set for a 15-megaton explosion (1, 2).] Results The Bravo bomb was the first thermonuclear weapon test using On August 1, 2018, we surveyed the depth of the Bravo crater solid LiD material as the central design for the fusion process, using a JMC V108 video echo sonar installed underneath the allowing this device to be aviation deliverable to enemy targets. The test was performed on a strip of land adjacent to Nam Island. The Bravo bomb was the second large-scale thermonu- Significance clear weapon test performed, following the first test, code named Ivy Mike, whose yield was 10.4 megatons. Ivy Mike was deto- High-yield thermonuclear explosions cause enormous radioactive nated in the northwest rim of Enewetak Atoll 16 mo earlier, on contamination to the environment. These “hydrogen bombs,” November 1, 1952. when tested on small islands in the ocean, vaporize the land and The Bravo bomb vaporized an artificial island, leaving a ∼75- produce radionuclides that settle in the ocean sediment. Even m-deep, 1.5-km-diameter crater located adjacent to a ∼0.25-km2 decades later, significant contamination may remain in the sedi- patch of land, called Nam Island (Fig. 1). Instantaneously, ment surface and deep into the sediment layers. Measuring the the Bravo test produced a blast, heat, and prompt radiation, which radioactive contamination of the crater sediment is a first step in dissipated rapidly (3). However, residual radiation from radioactive assessing the overall impact of nuclear weapons testing on the fallout (4) was spread throughout the world and, not surprisingly, a ocean ecosystems. We find radiation levels orders of magnitude particularly large concentration was deposited locally in the Bravo above background for plutonium-(239,240), americium-241, and crater in the northwest corner of the Bikini lagoon and throughout bismuth-207 in the top 25 cm of sediment across the entire Bravo the northern Marshall Islands. Residual radiation from such a bomb crater, the location of the largest aboveground US nuclear massive fallout can last from days to years, decades, centuries, and weapons test. beyond, depending on the half-life of the radionuclide. Nuclear weapons testing at the location of the Bravo crater did Author contributions: E.W.H., I.N.-H., and M.A.R. designed research; E.W.H. and M.R.M. performed research; E.W.H. and M.K.I.L.A. contributed new reagents/analytic tools; E.W.H., not end with the Bravo test. Six more nuclear weapons were M.K.I.L.A., and M.R.M. analyzed data; and E.W.H., I.N.-H., and M.A.R. wrote the paper. detonated in close proximity to the Bravo crater. Two tests in Reviewers: D.K., University of Massachusetts Amherst; and Y.G.K., University of particular, named Castle Romeo (detonated 1 mo after Bravo) California, Berkeley. and Hardtack Poplar (detonated in 1958), had yields of ∼10 The authors declare no conflict of interest. megatons each, and both were located southwest and approxi- This open access article is distributed under Creative Commons Attribution-NonCommercial- mately 2 km away from the Bravo center. These tests likely NoDerivatives License 4.0 (CC BY-NC-ND). knocked the southwestern rim of the initial Bravo crater, making 1To whom correspondence may be addressed. Email: [email protected] or mar@astro. the southern edge of the Bravo crater the deepest side. As a columbia.edu. result, today’s radioactive contamination from the Bravo crater Published online July 15, 2019. 15420–15424 | PNAS | July 30, 2019 | vol. 116 | no. 31 www.pnas.org/cgi/doi/10.1073/pnas.1903478116 Downloaded by guest on September 27, 2021 Fig. 2. Interpolated depth map of the Bravo crater based on echo sonar measurements (black dots). Fig. 1. Approximate location of the 7 tests conducted by the United States average values of 56, 34, and 10 pCi/g, respectively. The un- inside the Bravo crater. GPS coordinates collected from ref. 16. certainty in the measurements was less than 1 pCi/g in all cases. Before the primary radiation mapping project performed in SCIENCES Indies Surveyor August 2018, we collected 21 cores from 1 location at the Bravo ENVIRONMENTAL hull of a 23-m research vessel, , owned by the center in May 2017. The main goal of the 2017 coring pilot study Indies Trader company. Fig. 2 presents the results of an inter- was to investigate the reproducibility of the ocean sediment re- polated depth map of the crater by performing a kriging fit to the sults, when collected from precisely the same location. Table 2 sonar data using the R programming language (9). The fit used a presents the results for the mean and SD from the 5 radionu- Gaussian model (10) similar to an analysis done on island gamma clides studied for these 21 cores. The analysis of the cores by radiation measurements (11). A map generated from individual Gel Laboratories is identical to what was performed on the depth measurements, collected by the scuba divers, allowed for an 2018 cores. In fact, all cores were analyzed in the same batch independent assessment of the accuracy of the depth measure- submission in Fall 2018. In Table 2, we also list the radionu- ments. From this comparison, we estimate an overall uncertainty clide half-life, which has an effect on the current levels for a ± on our depth map to be 1.5 m. The main reason for the depth few of the radionuclides, given that the weapons testing occurred study was to aid in the dive planning. Sediment collection in the 65 y ago. majority of the crater necessitated deep diving to depths greater From August 2 to 9, 2018, 108 sediment cores were collected than 45 m, firmly in the range of advanced technical scuba diving. along 4 diameters across the crater, corresponding to the north– A total of 129 cores was collected in the Bravo crater over 2 y, south, east–west, northwest–southeast, and northeast–southwest in 2017 and 2018. Measurements were made of the following ra- directions. Fig. 3 presents a map of the designed 100-core plan 238 239,240 dionuclides: plutonium-238 ( Pu), plutonium-(239,240) ( Pu), versus the actual Global Positioning System (GPS)-measured 241 207 americium-241 ( Am), bismuth-207 ( Bi), and cesium-137 coordinates of the 108 cores that were collected.